Pirate, freebooter, buccaneer, privateer, corsair basically mean one who sails in search of plunder.
Pirate suggests a person or a ship or its crew that without a commission from an established civilized state cruises about in quest of ships to plunder. Since pirate in this sense is seldom used of contemporary life, the word has been extended to name one who wanders over a wide territory in search of plunder or one who infringes upon a right legally restricted to another or one known for predatory business practices.
Freebooter often suggests a maritime plunderer who pursues his occupation without the excuse that his country is at war and then differs from pirate only in its connotations of membership in a less closely organized band and of use of less violent methods. In extended use freebooter is often applied to one who seizes rights, privileges, and property on a large scale without regard to the restraints of law or of order.
Buccaneer, primarily applied to early French residents of Haiti, is more generally used of these people and others who preyed, sometimes with the tacit consent of their own governments, on Spanish ships and settlements in the New World. The term is often extended to an unscrupulous adventurer (as in business or politics); in such use it need not be wholly disparaging but does regularly imply disregard of the rules observed by ordinary men.
Privateer and corsair primarily apply to a ship privately owned but commissioned by its government (as in the 17th and 18th centuries) to prey upon other ships, usually those of an enemy, but in practice either term may designate a ship, its commander, or one of its crew. Corsair is applied chiefly to a ship, a commander, or a sailor of North African origin. Neither term has extensive extended use, but when so used they are quite distinct: privateer then applies to one doing in a private capacity what would normally be undertaken by a public official, but corsair attributes fury and rapacious cruelty to the one so-called.